Reductio ad absurdum

In logic, Reductio ad absurdum (Latin: "reduction to the absurd") also known as an apagogical argument, reductio ad impossibile, or proof by contradiction, is a type of logical argument where one assumes a claim for the sake of argument, derives an absurd, ridiculous or contradictory outcome, through logical progression, and then concludes that the original assumption must have been wrong as it led to an absurd result.

It makes use of the law of non-contradiction — a statement cannot be both true and false. In some cases it may also make use of the law of excluded middle — a statement must be either true or false. The phrase is traceable back to the Greek "ἡ εἰς ἄτοπον ἀπαγωγή" (hē eis átopon apagōgḗ), meaning "reduction to the absurd", often used by Aristotle.